Are you reading 2 piece?
Coby literally couldn't get past the Boa sisters, didn't even fight Hancock and he was still bleeding like a fountain.
Blackbeard was the one who did anything against Hancock not coby
I get the point u can be the strongest with just haki but not just with abilities, but do you think if luffy had a bit better haki he would've won against kaido ? Devil fruit adds way more then just flat out power like haki, This argument about haki transcends cant be fully correct if you have people like whitebeard or rocks (i think he got fruit as well) that were able to rivel roger. only people in the story that can really "win" against devil fruits are roger and garp.
Take law for example, he went against haki master vergo and he won because of his devil fruit, also i think blackbeard will kill shanks and if he will that will be because of his df not his haki.
why does that matter to my point ? my point is THE STRONGEST MARINE is the PIRATE KING's rival and that's just Sakazuki BY FAR , unless you think Smoker/Koby jump 20 levels in few months , they CANT compete